I totally understand your point and I feel that for the most part, you're right. Gotham has become a city where the people are just again, the people are active again and they will remember Batman's legacy and not allow crime to take control of the city once more.
However, Bruce, being the tactician that he is, plays all the angles. And while I'm sure he is proud and satisfied that he's done the job he set out to do, when he goes off to divest himself of Gotham (which in a way has been holding him back for so very long) he's still going to make sure that, should there be something that the citizens of Gotham cannot handle by themselves, Batman will still be there to protect the city. So while Gotham may not need Batman, certainly not in the way it did in Begins, Bruce see's a kindred spirit in Blake, someone who can take up the mantle and be there for the city if something comes up that is beyond the capabilities of the police and citizenry.